Thermo Scientific TVA2020 Introduction

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Lindsey Campbell
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1 Thermo Scientific TVA2020 Introduction The Thermo ScientificT TVA2020 Toxic Vapor Analyzer, is an advanced design, portable, organic/inorganic vapor monitor for the gas survey industry. The analyzer uses either a flame ionization detector (FID), or both an FID and a photoionization detector (PID). Flame Ionization Detection (FID) A Flame Ionization Detector (FID) measures organic compounds by utilizing a flame produced by the combustion of hydrogen and air (see Figure 1-1). When hydrocarbons in the sample are introduced to the detection zone, ions are produced by the following reaction: where: RH + O RHO + + e - H2O + CO2 R = carbon compound A collector electrode with a polarizing voltage is also located within the detector chamber, and the ions produced by this reaction are attracted to it. As the ions migrate towards the collector, a current is produced which is directly proportional to the concentration of hydrocarbons introduced to the flame. This current is then amplified and sent to a microprocessor. The FID has a wide dynamic range. The effective dynamic range can be further expanded by use of a dilutor kit which reduces very high volatile organic compound (VOC) concentrations to within the dynamic range (or even linear range) of the analyzer. The dilutor kit can also be used to enrich oxygen deficient samples by adding ambient air that is rich in oxygen (20.9% usually). Low oxygen can affect the characteristics of the hydrogen flame, causing readings to be artificially elevated and possibly extinguishing the flame. As a general rule of thumb, greater than 16% oxygen is required to support the flame. If underground gases or samples in gas bags are to be measured by an FID, it is advised that a dilutor be used to combat the problem.

2 Figure 1-1. Typical Flame Ionization Detector Benefits of Flame Ionization Detection Benefits of Flame Ionization Detection include: Wide dynamic and linear range High sensitivity to hydrocarbon vapors (including methane) Very stable and repeatable response Virtually unaffected by ambient levels of CO, CO2, and water vapor

3 Photoionization Detection A Photoionization Detector (PID) consists of an ultraviolet (UV) lamp of a specific energy and an ionization chamber (see Figure 1-2). Compounds passing through the chamber are excited by photons of UV energy and ionized according to the following equation: where: R + hυ R+ + e R = most organic/inorganic compounds These ions are attracted to a collecting electrode, producing a current proportional to the concentration of the compound. Whether or not a compound can be detected by a PID depends upon the energy required to remove an electron from the compound (its ionization potential). If the lamp energy is greater than the compound's ionization potential, the PID will detect it. The lamp in the TVA2020 is 10.6 ev. Because of its smaller dynamic range ( ppm), the PID is not the detector of choice for measuring high concentrations of vapors. A PID is also more susceptible to interference from water vapor than a FID. However, as a PID does not require hydrogen or oxygen, it is the detector of choice when fuel is limited or unavailable, or when ambient oxygen concentrations are low. The PID is also very sensitive to aromatic and chlorinated compounds, and can even measure some inorganic compounds that the FID does not detect at all (ammonia, carbon disulfide, carbon tetrachloride, chloroform, ethylamine, formaldehyde, and hydrogen sulfide, to name a few).

4 Figure 1-2. Typical Photoionization Detector Benefits of Photoionization Detection Benefits of Photoionization Detection: High sensitivity to aromatics, unsaturated hydrocarbons and chlorinated hydrocarbons Ability to measure some inorganic gases Very simple operation No support gases required Non-destructive detector allows sample to be recovered

5 Dual Detectors The benefits of each individual detector are very clear: both the FID and the PID have their advantages and disadvantages. However, with either detector alone, the number of organic and inorganic vapors that one can detect is limited by the measurement capabilities of that detector. With the TVA2020, users can obtain more complete information about organic and inorganic vapors more quickly and easily than with single detector technology alone. Since both detectors may be displayed and logged simultaneously, the relative response of the two detectors may give some clues about the identity of the compound being measured (see Figure 1-3). For instance, the PID does not respond to methane at all, but the FID responds very well. A high FID reading with virtually no PID response might indicate the presence of methane. Consequently, PIDs respond very well to some inorganic gases that FIDs cannot detect. A high PID reading with no FID reading might suggest the presence of an inorganic compound. With readings from both detectors readily available, the TVA2020 can help a user make decisions about the type of compound present and which detector reading to use.

6 Figure 1-3. TVA2020 Dual Detector Configuration Benefits of Dual Detectors Benefits of Dual Detectors: Cost-effective packaging Detector response ratios can help characterize compounds Enhanced analytical capability derived from simultaneous detection

7 Concentration Calculation and Calibration The concentration calculation is defined as the process that transforms a detector count into a displayed final concentration. The process is as follows:

Title: Standard Operating Procedure for Measurement of Total Hydrocarbon Using a Flame Ionization Detection

Title: Standard Operating Procedure for Measurement of Total Hydrocarbon Using a Flame Ionization Detection Procedure No: SOP-020 Revision No: 1.0 (January 21, 2011) Page No.: 1 of 7 1. INTRODUCTION AND SCOPE To obtain timely data for the purpose of air quality assessment, air quality trend reporting, and to